1. Field of the Invention
The invention generally relates to the field of signal processing for wireless communications. More specifically the invention is related to efficient projections of signals and variations thereof for the purpose of reducing the effects of interference.
2. Discussion of the Related Art
Multipath and other forms of interference inherently limit the performance and capacity of wireless communication networks. Common wireless transmission protocols, including Code Division Multiple Access (CDMA) and Wideband CDMA (W-CDMA) are interference-limited both in uplink and downlink communications.
Advanced signal processing at the receiver can mitigate interference and thereby increase network capacity and coverage. A well-known interference-mitigation technique employs an orthogonal projection canceller configured to cancel selected interference from a received baseband signal.
FIG. 1A shows a baseband portion of a prior-art Rake finger configured to project out interference from a received baseband signal. A projection canceller 102 precedes a baseband front-end 103, a descrambler (e.g., PN descrambler 105), and a despreader (e.g., Walsh despreader 107). An RF front-end (not shown) and a matched filter (not shown) typically precede the projection canceller 102. The baseband front-end 103 may include a delay compensator (not shown), a chip-rate sampler (not shown), and a serial-to-parallel converter (not shown) for producing a digitized baseband signal.
Symbols on interfering subchannels and multipaths are identified, estimated, and used to synthesize an interfering signal, such as shown in FIG. 2A. An interference selector 209 identifies which subchannels potentially interfere with a signal of interest and produces symbol estimates for combinations of selected interfering subchannels and multipath components. A transmission synthesizer 211 uses the symbol estimates and subchannel information to synthesize transmitted interference. The synthesized transmitted interference may be distorted by a channel emulator 213 configured to reproduce channel distortions measured from the received signal. The channel emulator 213 output is processed by a projection canceller 202 configured to project the received baseband signal onto a subspace that is orthogonal to an interference space generated by the synthesized interfering signal.